Generally speaking, a cardiac pacemaker is an electrical device used to supplant some or all of an abnormal heart's natural pacing function, by delivering appropriately timed electrical stimulation signals designed to cause the myocardium of the heart to contract or "beat". Stimulation signals usually have well-defined amplitude and pulse width characteristics which can be adjusted to meet physiologic and device power conservation needs.
The strength (amplitude) and duration (pulse width) of the stimulation signals must be of such magnitude that capture is maintained, to prevent serious complications and even death. Yet, it is desirable for these magnitudes not to be higher than is needed for a reasonable safety margin for longer battery life. Chief among the problems is that stimulation signal thresholds necessary for maintaining capture often fluctuate in the short term, and gradually change in the long term. It has been clinically observed that the lowest threshold is observed immediately after implantation of the pacemaker (the acute threshold). Inflammation in the tissue around the tip of the stimulation electrode requires greater energy to propagate the stimulation signals, thereby driving the threshold up sharply during the first two to six weeks to its highest level (the peak threshold). Some of the inflammation reduces over the long-term, to lower the threshold below the peak level-the chronic threshold. However, the chronic threshold does not reduce to the acute level, since some permanent fibrous tissue, requiring greater energy than non-fibrous tissue for signal propagation, remains around the electrode tip. In the short-term, thresholds may decrease with exercise, for example, and may increase with various activities, including sleep.
Some prior art implantable pulse generators (IPGs) which serve as cardiac pacemakers have an automatic capture feature to maintain capture or restore capture after a loss-of-capture episode, without the need for clinical or patient intervention. In addition, some of these IPGs have an automatic threshold-seeking feature, which, either after capture restoration or periodically, seek the lowest "safe" voltage level of the stimulation signal for energy efficiency. That is, the voltage of the stimulation signal is lowered to the newly detected threshold voltage plus a safety margin, rather than using an unnecessarily high stimulation signal voltage level.
During the search for thresholds, some prior pacemakers deliver a backup pulse (as a safety measure) after a primary pacing pulse in case the primary pacing pulse fails to capture the patient's heart. The backup pulse is delivered after the passage of a predefined Vulnerable Period, as is well known in the art, to avoid facilitating re-entrant tachycardia and fibrillation. As a result, the aforementioned prior art pacemakers result in a drop in pacing rate and long threshold searching times. The patient may be aware of either the drop in pacing rate, or alternatively an artificially elevated rate used to compensate for the rate drop.